(1) Field of the Invention
This invention relates to highly durable silica glass, a process for producing the silica glass, a member comprised of the silica glass, and an apparatus provided with the member.
The silica glass of the present invention has enhanced durability, and good processability and machinability, and therefore, a member comprised of the silica glass is suitable for apparatuses using a plasma, such as a semiconductor-producing apparatus and a liquid crystal-producing display apparatus.
(2) Description of the Related Art
In fields of production of semiconductor or liquid crystal, production apparatuses using a plasma are widely used. In recent years, semiconductor integrated circuits have become minute, and hence, a dry-etching step using a plasma bears importance. In the dry-etching step, a halogenated gas such as fluorine-containing gas or chlorine-containing gas is used.
The halogenated gas and its plasma have a high reactivity and thus are utilized in various steps, for example, as an etching gas in an etching stop or cleaning gas in heat CVD step. The halogenated gas used includes, for example, fluorine-containing gases such as F2, HF, CF4, C2F6, C3F8, C4F8, CHF3, SF6 and NF3; chlorine-containing gases such as Cl2, HCl, BCl3 and CCl4; and bromine-containing gases such as Br2 and HBr. In recent years, gases having a low globe-warming factor, such as C5F8 and C4F6, have been proposed (For example, Toru Ida et al: Semi-Con., Japan 2002, NAVIGATOR, p39–49, 2002).
Inside the semiconductor production apparatus and liquid crystal display production apparatus, a silica glass member is provided. This is because silica glass has a high-frequency transmittance which is important for plasma generation, and silica glass can be shaped into a member of a complicated shape with high purity at a low production cost. The reason for which silica glass member of a complicated shape can be produced at a low cost, is that a rock crystal powder of a high purity used as a raw material for silica glass is inexpensive, and further that it has good processability and machinability, i.e., machining can be easily carried out and oxyhydrogen flame-welding can be conducted.
Silica glass has good characteristics, as mentioned above, but has problems. That is, a surface of a silica glass member is gradually etched and eroded by a halogenated gas or its plasma at the exposed location. This undesirable etching reduces the durability of the silica glass member and sometimes causes abnormal discharge.
To remedy the above-mentioned problems, it was proposed that the members to be exposed to a plasma are composed of a sintered body exhibiting enhanced resistance to etching. Such a sintered body includes, for example, a sintered alumina body exhibiting a lower etching rate when exposed to a plasma, than that of silica glass, was proposed.(Japanese Unexamined Patent Publication [hereinafter abbreviated to “JP-A”] H5-217946), a sintered body comprising yttrium-aluminum-garnet (JP-A H10-236871), and a sintered aluminum nitride body (JP-A H10-275524). However, these sintered bodies have problems such that a raw material with a high purity is difficult to prepare, and, they have a grain boundary and therefore, when the sintered body is etched, the particles are liable to be separated from the sintered body with the result of reduction in yield of semiconductors or liquid crystal displays. Further, they have poor processability and machinability, and the produced members are costly.
An attempt of adding a corrosion resistant element in silica glass is made to reduce the undesirable etching caused by a plasma while the good properties inherently possessed by silica glass, such as high purity, low cast, good processability and low dusting property are kept. More specifically, an attempt is made of adding an element in silica glass, a halide of which has a sublimation temperature or a boiling point higher than that of a halide of silica contained in the silica glass, whereby a silica glass composition exhibiting a reduced etching rate is provided.
For example, a proposal was made of incorporating a metal in silica glass, fluoride of which metal has a boiling point higher than that of fluoride of silica, whereby a silica glass composition containing bubbles and foreign matter in amounts such that the projected area of glass is smaller than 100 mm2 per 100 cm3 is provided (JP-A 2002-137927).
A proposal was made of incorporating in silica glass at least one metal element selected from Sm, Eu, Yb, Pm, Nd, Ce, Tb, Gd, Ba, Mg, Y, Tm, Dy, Ho, Er, Cd, Co, Cr, Cs, Zr, Al, In, Cu, Fe, Bi, Ga and Ti, to provide a silica glass composition containing bubbles and foreign matter in amounts such that the projected area of glass is smaller than 100 mm2 per 100 cm3, and having a OH concentration of 100 to 2,000, a surface roughness of 0.01 to 10 μm, and a gas emission of at least 2 mol/m3 as measured when the temperature is elevated from room temperature to 1,000° C. (JP-A 2002-193634).
However, the inventors have found that the addition of the above-mentioned anticorrosive elements exerts a function of lowering the etching rate, but tends to destroy the structure of silica glass which offsets the durability enhancement function performed by the addition of corrosive elements. The reason for which the structure of silica glass tends to be destroyed is believed that the SiO2 network constituting silica glass is broken by the incorporation of secondary elements, and consequently, non-crosslinking oxygen having a weak bonding force is introduced. This undesirable phenomenon occurs remarkably under etching conditions in which incidence of ion with high energy occurs markedly. Thus it was possible that, by the addition of secondary elements, the etching rate of silica glass is rather increased and the durability is reduced.
As a solution of the above-problem, a glass composition composed of SiO2—Al2O3—CaO, siO2—Al2O3—MgO, SiO2—BaO—CaO or SiO2—ZrO2—CaO or SiO2—TiO2—BaO was proposed (for example, JP-A 2002-121047). These proposed glass compositions are not advantageous because these contain repellent elements for the semiconductor production such as Mg and Ca or toxic elements such as Ba.
As another solution, a glass composition composed of Nd2O3—SiO2—Al2O3 or Dy2O3—SiO2—Al2O3CaO was proposed (for example, Japanese Patent No. 3261044). These proposed glass compositions also are not advantageous because an expensive raw material such as Nd or Dy is used in a large amount and these glass compositions are generally difficult to vitrify.
A further solution was proposed wherein a silica glass composition comprising 0.1 to 20% by weight of a combination of a first metal element selected from Group 3B elements of the Periodic Table with a second element selected from Zr, Y, and lanthanoid and actinoid elements (for example, JP-A 2002-220257). This glass composition has a problem such that, when a large amount of elements are added, cracks, bubbles and turbidity tend to occur, although glass state (i.e., amorphous structure) can be kept.